kirowan's page

NPC classes are far less powerful than PC classes.

NPCs - even those with PC classes - have less spendable monetary resources for item optimization than PCs of a similar level.

Regardless of class, all NPCs tend to use a lower stat-array system than a PC's point-buy/average dice roll usually produces. (That said, this really isn't that big a difference.)

These things combine to create weaker creatures on-average.

The CR is a combination of these rough generalities to give you the gist of an idea.

Ok, thanks. However,I did not find such a info in the Corebook (at least regarding the spendable money and the related low CR)

It's supposed that surfaces greater than 3 feet wide are any floor, and tge base DC for Acrobatics is 0. However, if that floor is covered with gravel (slightly obstructed), the DC is +2. Such a surface (gravel) is difficult terrain and the movement on them is halved. Moving at full speed on uneven surfaces is DC +5. So, it is DC 7 to move at full speed on gravel. BUT no skill check is needed to move across such surfaces unless the DC is 10 or greater. That means: you can always move on gravel without penalties on your movement. Am I OK?
Thanks!